The present invention relates to an integrated circuit overload protection device.
When an integrated circuit common emitter amplifier is driven by an input signal, the polarity of the output signal is inverted relative to the input signal. Referring to FIG. 1, a schematic diagram for a common emitter amplifier circuit of the type known in the prior art, is shown. In a typical configuration, the common emitter amplifier comprises an NPN transistor Q. The collector of transistor Q is connected to the positive power supply terminal, +V, through a load resistor, R.sub.L. The emitter of transistor Q is connected to ground through an emitter resistor, R.sub.E. An input signal, v.sub.i, is coupled to the base of transistor Q through an input resistor, R.sub.S. The resulting input base current causes a collector current, equal to beta times the input base current, to flow. Consequently, the output voltage v.sub.o, measured between the collector of transistor Q and ground, will be an amplified and inverted version of the input voltage, v.sub.i.
The emitter voltage of a common emitter amplifier which utilizes an emitter resistor, R.sub.E, tries to follow the input voltage in the so-called "emitter follower" mode. Thus, while the output signal, v.sub.o, is being driven down by an increasing input signal, v.sub.i, the emitter voltage is increasing. Depending upon the relative values of the circuit elements and the amount of input drive available, it is possible for Q to saturate and for there to be enough drive present for the emitter voltage to dominate the collector voltage during input peaks. Thus, while the input signal (solid line adjacent input) shown in FIG. 1 should result in the output signal (solid line adjacent output) shown, it is possible for the most negative portion of the output signal to be replaced by the portion shown in dashed line. Such result is most apt to happen under conditions in which R.sub.S is small and a large drive signal is present.
In the case of a negative feedback amplifier, in which a portion of the output signal is fed back into the input in order to help linearize the amplifier's transfer characteristics, it is possible, during an overload condition, for the amplifier to operate as a positive feedback amplifier. In video amplifiers, of the type used in television circuits, the over driving of a video amplifier preceding the automatic gain control (AGC) circuit, can result in the phenomenon known as "lock out", resulting in a garbled picture. However, it should be recognized that the problem described above is common to many common emitter amplifiers, including those used in differential and operational amplifiers. Accordingly, a protective device which would prevent this phenomenon from occuring would be highly desirable.